With regard to conventional apodized interdigital transducers, methods are known of suppressing undesired reflection (hereinunder referred to as "MEL" (Mass Electrical Loading). Such MEL is caused by the difference in the characteristic impedance to the propagation of surface acoustic waves between a portion where there is an electrode and a portion where there in no electrode. One of the methods is described in "The IEEE Trans. MTT-22" (1974), pp 960 to 964 in which MEL is suppressed by making the width of an electrode equal to 1/8 of the wave length (.lambda..sub.0) of a surface acoustic wave at the center frequency (f.sub.0).
In the above-described prior art, however, no consideration is given to an irregular pitch electrode group in which the frequency characteristic of amplitude is non-symmetrical, in other words, the pitch of the sources of surface acoustic wave is not constant so that the suppression of the MEL is insufficient.
Although it is possible to suppress MEL by making the width of an electrode and a non-metalized part 1/8 of the intercentral distance between the adjacent sources of surface acoustic wave even when the frequency characteristic of amplitude is non-symmetrical and the intercentral distance between the adjacent electrodes is not constant, the above-described conventional method is insufficient for suppressing MEL when the asymmetry is too large or sufficient suppression of reflection is required.